The invention relates generally to the field of dipole antennas which are used for radio transmission and reception and which use a conductive container for each of the two dipole radiating elements. These containers are spatially separated and typically the R. F. impedance between them is very high. The dipole elements are excited (fed) by applying therebetween a radio frequency (RF) potential at the desired operating frequency. One advantage of such an antenna is that associated radio components can be mounted inside the conductive containers and therefore are shielded from external capacitances and radiation. Another basic advantage of such a system is that the conductive containers can now serve as part of the external casing of a radio device (transmitter and/or receiver). Thus the need for a separate antenna structure in addition to the radio casing is eliminated and an overall size reduction is obtained.
When a small size radio device is desired, such as in a portable pager, electrical components must be mounted in both conductive containers and interconnecting wires must be provided between the conductive containers. Normally these wires provide interconnecting paths for signals, such as audio and D. C, having frequencies substantially below that of the RF signal to be transmitted (or received).
The radio components (apparatus) in each container will tend to float at the RF potential of their respective containers and hence the interconnecting wires form parallel (shunt) RF impedance paths between the dipole radiating elements. To provide RF isolation between the radiating elements, RF chokes are usually connected in series with these wires. These chokes are effectively connected between the dipole elements and normally must have a high Q value or else the interconnecting wires will seriously load the dipole antenna and decrease its efficiency. The RF impedance which exists, in prior systems, between the points where the isolation chokes are connected is usually not controlled or even considered, and therefore the RF chokes may be connected across points having an extremely high RF impedance therebetween. Thus even high Q chokes may create a serious shunt low impedance path between the radiating elements of the dipole. Prior art dipole antenna systems provide no suitable and predictable RF impedance points for connecting RF isolating chokes. Therefore, very high quality (Q) RF chokes are required to minimize loading effects.
The antenna input impedance between the dipole radiators must be matched to the transmitter (or receiver) impedance. This is usually accomplished by a complex impedance matching network in addition to any associated mechanical support structure, such as an insulated conduit, for the interconnecting wires.
This mechanical support, which may provide some shielding for the wires, also creates an additional shunt R. F. impedance between the dipole elements and therefore decreases the antenna system efficiency. The size and complexity of the prior art systems also suffer because both an impedance matching network and a separate mechanical support structure are used.